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Harking back to a simpler time, when technology was evident, and worked!
By Jason Griffiths
To the east of Johnson City, Texas, is the Lyndon B. Johnson family home. Part of the Johnson Estate is given over to a working farm circa 1870 that presents various aspects of domestic practice from the era. This includes a desert fridge, a simple four-legged structure with a slightly battered profile that’s draped in cloth. Its principle is simple: water from an inverted jar is drawn by osmosis down the sides of the cloth where it evaporates in wind currents that blow through a “dog run” between two log cabins. Cooled air circulates within the structure where cheese and milk are kept fresh during the summer. The desert fridge is a simple system that reaches a state of equilibrium through the natural process of evaporation.
This system provided a working model for a prototype structure for an outdoor dining pavilion that was designed and constructed on the campus of Arizona State University in spring 2008. The pavilion is intended as a gathering point and a place of interaction for ASU freshmen.
Upturned jars and an inverted table cloth
For invited freshmen, a dining event is preceded by an invitation that is sent in a simple glass jar. The jar contains instructions printed on a clear plastic sheet that describe not only the event but also what to do with the jar. Once the jar is inverted, water gradually seeps out under osmotic pressure through the cloth. This cloth, like the inverted jar, shares affiliations with a formal dining ritual. It can be removed and replaced like a simple tablecloth.
At the center of the table is an aluminum fan that hangs below the surface of the table, driven by a roof-mounted 50 watt direct current PV panel that forces the air downward onto a diverter and then horizontally through the inverted tablecloth. Air is cooled through the evaporative process and then blown onto the legs and midriffs of the students dining at the table. The dinner continues and the students remain in the cooler environment as the water level in the jars gradually recedes in front of them. An analysis of the system demonstrates a considerable drop-off in ambient air temperature up to 1.8m from the table’s center. This analysis is based on a 40.6cm fan generating airflow rates approximately 944 L/s operating at 1,500 rpm. Air is blown equally in four directions below the table through an evaporative cooling membrane of heavy cotton fabric (total area is 2.16m2) with strands removed to provide greater percentage of perforation. Our working model studies* revealed that the most effective quality of perforation was around 30% of the fabric. These factors produced more than an 11C difference between the table center and a distance of 1.8m in an ambient temperature of 54C and during relative still air conditions. These temperatures were highly affected by wind currents around the pavilion, being considerably cooler sitting in a downwind position. The scheme provides a cooled microclimate around the pavilion that is designed to sustain conversation, prolong the event and encourage student interaction.
Jason Griffiths is an assistant professor in architecture at Arizona State University, Tempe.
Notes
* Initial working model was based on a simple soaked cloth and re-arranged house fan.